Multi-tiered-recess screws typically include a driving recess for insertion of a driving tool, which recess includes a plurality of superimposed recess tiers of decreasing size. The tiers may be concentric, in which event they are non circular. The invention is particularly concerned with the latter, because these have the additional feature that, whereas the driver has a fixed number of tiers, the screw may have some or all of the tiers, depending on its size and torque driving requirements.
It is a particular feature of this kind of screw that there is a single driver that is suitable for driving a range of sizes of screw. Smaller screws simply have one or two small recesses, while the larger screws have larger recesses also.
GB-A-1150382 appears to be the first disclosure of a screw provided with a multi-tiered recess and a corresponding multi-tiered driver. GB-A-2285940 discloses essentially the same idea. Both these publications describe the advantages provided by the arrangements disclosed. The first is that the recesses are essentially parallel-sided and consequently eliminate cam-out problems that are associated with cross-head recesses. Secondly, they give the possibility of a single driving tool being suitable for driving a wide range of screw sizes.
The single driving tool typically has three (for example) tiers of driving surfaces which are employed to drive large screws having three tiers of recess. However, the same tool can be employed with smaller screws having only two tiers of recess, the largest tier being omitted. Indeed, even smaller screws may have only one, the smallest tier, in their recess and be driven by the smallest tier only of the tool.
GB-A-2329947 discloses a similar arrangement, and WO-A-0177538 discloses tiers that have such a small extent in the recesses of screws and bolts that, at the torques at which the screws are intended to be operated, they cannot be turned unless at least two tiers are both engaged by the tool. Otherwise, the screw is arranged to round out of engagement with the driving tool. This provides a security feature in that only the appropriate tool will undo the screw.
WO03/025403 discloses a method of manufacture of such screws using cold forming punches. It is possible to make the recesses with some precision, so that the driving tool is a close fit in the recess. This has the very useful feature that tiers can be shallow. Then, screw heads do not need to be large to accommodate the driving tool. Yet, adequate torque can still be applied because a large proportion of the area of each recess is used for torque transmission by virtue of the close tolerance fit. But, equally usefully, the tool fits the screw so closely that, once mated with the driving tool, the screw can be carried solely by the driver when it is offered up to a workpiece. Indeed, with self-tapping wood screws, the connection between driver and screw is so stable that some pressing and simultaneous rotation forces can be applied to the tool, without holding the screw. This can be done without significant risk that the connection will fold as frequently happens, unless forces are maintained absolutely axial. It cannot be done at all with, for example, slotted screws. This feature is almost without limitation. Certainly, with three-tiered screws, such as those available under the trademark UNI-SCREW, the fit is so close that even carrying screws dangling vertically from the driver still fail to fall off if carried carefully.
Nevertheless, it would be desirable to improve this “stick-fit” feature. This is particularly so with screws having only one or two tiers of recess. The smallest recess is not quite such a close fit, and it seems that it is partly the plurality of tiers that, at least to some extent, explains why the screw appears to grip the driver so effectively. So, with smaller screws having just one tier of recess the stick-fit feature is not so evident. It is an object of the present invention to improve the stick-fit feature of screws to which this patent application relates.